Camshaft phasers for varying the timing of combustion valves in internal combustion engines are well known. A first element, known generally as a sprocket element, is driven by a chain, belt, or gearing from the crankshaft of the internal combustion engine. A second element, known generally as a camshaft plate, is mounted to the end of a camshaft of the internal combustion engine. A common type of camshaft phaser used by motor vehicle manufactures is known as a vane-type camshaft phaser. U.S. Pat. No. 7,421,989 shows a typical vane-type camshaft phaser which generally comprises a plurality of outwardly-extending vanes on a rotor interspersed with a plurality of inwardly-extending lobes on a stator, forming alternating advance and retard chambers between the vanes and lobes. Engine oil is supplied via a multiport oil control valve, in accordance with an engine control module, to either the advance or retard chambers, to change the angular position of the rotor relative to the stator, and consequently the angular position of the camshaft relative to the crankshaft, as required to meet current or anticipated engine operating conditions.
While vane-type camshaft phasers are effective and relatively inexpensive, they do suffer from drawbacks such as slow operation at low engine speeds due to low oil pressure, slow operation at low engine temperatures due to high oil viscosity, increased oil pump capacity requirement for the oil pump used to lubricate the internal combustion because the same pump is used to actuate the vane-type camshaft phaser, and the total amount of phase authority provided by vane-type camshaft phasers is limited by the amount of space between adjacent vanes and lobes and may not be sufficient to provide the desired amount of phase authority. For at least these reasons, the automotive industry is developing electrically driven camshaft phasers.
One type of electrically driven camshaft phaser being developed uses a harmonic gear drive unit, actuated by an electric motor, to change the angular position of the camshaft relative to the crankshaft. One example of such a camshaft phaser is shown in U.S. Pat. No. 8,322,318 to David et al., the disclosure of which is incorporated herein by reference in its entirety. The camshaft phaser of David et al. includes stop members which limit the phase authority of the camshaft phaser, i.e. the extent to which the camshaft phaser is able to advance and retard the camshaft relative to the crankshaft. While the camshaft phaser of David et al. may be effective, other options may be desirable to offer design flexibility and meet requirements in different vehicle applications.
What is needed is an electrically driven camshaft phaser which minimizes or eliminates one of more of the shortcomings as set forth above.